1. Field of the Invention
The invention relates to a positive-electrode plate for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery including the positive-electrode plate, and a method of producing the non-aqueous electrolyte secondary battery.
2. Description of Related Art
There have been demands for lithium-ion secondary batteries with improved performance, such as, higher-power lithium-ion secondary batteries and higher-capacity lithium-ion secondary batteries. From the viewpoint of producing high-performance lithium-ion secondary batteries, active materials with a high electric potential, such as Ni—Mn spinel-based materials, have drawn attention as positive-electrode active materials. This is because using positive-electrode active materials with a high electric potential leads to a raise in the operating voltage of lithium-ion secondary batteries.
In a lithium-ion secondary battery, as the operating voltage thereof becomes higher, decomposition of an electrolyte is more likely to occur with charging and discharging of the lithium-ion secondary battery. In the lithium-ion secondary battery in which acid is generated due to decomposition of the electrolyte, a transition metal may be eluted from a positive-electrode active material due to the acid. When the transition metal is eluted from the positive-electrode active material, the capacity maintenance ratio of the lithium-ion secondary battery is lowered.
For example, Japanese Patent Application Publication No. 2014-103098 (JP 2014-103098 A) describes a non-aqueous electrolyte secondary battery including a positive-electrode active material layer that contains a positive-electrode active material with a high electric potential and trilithium phosphate (Li3PO4). According to JP 2014-103098 A, trilithium phosphate contained in the positive-electrode active material layer makes it possible to prevent elution of a transition metal from the positive-electrode active material with charging and discharging of the non-aqueous electrolyte secondary battery. Specifically, trilithium phosphate is reacted with hydrofluoric acid (HF) generated in the electrolyte, thereby functioning as an acid-consuming material. This makes it possible to inhibit elution of the transition metal from the positive-electrode active material, thereby enhancing the durability of the non-aqueous electrolyte secondary battery.
In order to allow trilithium phosphate contained in the positive-electrode active material layer to appropriately react with hydrofluoric acid, trilithium phosphate needs to be present near the sites where hydrofluoric acid is generated. In other words, it is desirable that trilithium phosphate be appropriately distributed without imbalances in the positive-electrode active material layer. However, in the foregoing related art, the distribution of trilithium phosphate in the positive-electrode active material layer is not appropriate in some cases. In these cases, trilithium phosphate is not present in many of the sites where hydrofluoric acid is generated. As a result, there is a large amount of hydrofluoric acid that does not react with trilithium phosphate.